(i) Field of the Invention
This invention relates to load cells, which are especially useful for wheel load scales and weigh-in-motion scale.
(ii) Description of the Prior Art
There are many types of load cells. For example, some types of load cells are operable in response to changes in reluctance of magnetic circuits, such load cells being capable of measuring compressive forces only, tensile forces only, or both compressive and tensile forces. Known such types of load cells typically consist of a rigid steel beam supported at its opposite ends in a manner such that its mid-section may deflect downwardly in response to a weight load from the weighbridge platform.
The load cell typically includes a number of strain transducers, generally referred to as strain gauges, affixed to its lower surface. Downward deflection of the load cell is manifested by tensile stresses in the lower surface of the load cell. The strain gauges respond to such stresses by producing an electrical signal that varies in magnitude with the extent of the load cell deflection. The load cell and its strain gauges are calibrated such that the assembly can be used accurately and reproducibly to measure weight loads on the load cell. In operation of the platform scale, the electrical outputs from the tow load cells under the opposite ends of a weighbridge platform are combined electronically to obtain a measure of the total weight load on the platform.
Many patents are directed to weighing scales which make use of the above-described type of load cells. For example, U.S. Pat. No. 4,261,429, issued Apr. 14th, 1981, to H. E. Lockery, provided a scale including a platform and transducer elements which are combined so that the transducer elements form integral components of the platform proper. For this purpose slots were milled, machined or cut into the platform so that two parallel slots defined a beam structure, the sensitivity of which was established by drilling holes into the platform, at the end of each slot and intermediate the ends of each slot. Strain gage elements were then secured to the so-formed beam structures, the free ends of which were operatively connected to support elements. Mounting means, preferably of resilient material, were secured through spacer elements to the free end of the beam structures serving to support the weighing apparatus on the mounting surface. Overload limiting means were so arranged that the flexing of each beam structure is limited to a predetermined value.
U.S. Pat. No. 4,565,255, patented Jan. 21st, 1981, by M. Serrazin, provided a weighing device in which the opposite ends of a metal bar placed between a weighing platform and a base plate were rigidly fixed respectively to the platform and to the base plate. The metal bar carried flexure-sensitive strain gauges and torsion-sensitive strain gauges for cancelling the torsional stresses detected by the gauges. Two pairs of flexure-sensitive strain gauges were mounted on one face of the bar and aligned along the bar axis. The gauges of each pair were connected in opposition of sign in a Wheatstone bridge circuit and an electric signal, which was proportional to the force applied on the weighing platform, was measured at the bridge terminals.
U.S. Pat. No. 4,581,948, patented Apr. 15th, 1986, to K. W. Reichow, provided a load cell assembly which included a deflectable load cell having strain transducers which measured the downward deflection of the load cell in response to a load. The load cell included grooves in the lower surface thereon in the vicinity of each end thereof. A rocker pin was positioned in each groove, the groove being configured in such a manner that the pin was captured therein, so that the pin in the groove acted in operation to restrain the load cell, but so that the pin extended below the lower surface of the load cell. The pin was generally barrel-shaped, having its greatest dimension at its centre, so that the load cell was supported on a small area of the pin, generally about the centreline of the load cell. The load cell assembly also included two end mounts for supporting the respective ends of the load cell. The load cell assembly also included two end mounts for supporting the respective ends of the load cell, each end mount including means for receiving the ends of the rocker pins, which extended beyond the side surfaces of the load cell.
U.S. Pat. No. 4,666,003, patented May 19th, 1987, by K. W. Reichow, provided a load cell for on-board weighing applications, including an elongated sheer force measuring beam which included mounting means for securing the opposing longitudinal ends of the beam to the frame of the vehicle. The load was applied against the upper surface of the beam through a platform. The shear strains caused by the load were concentrated in an area near each end of the beam. The strain measuring means were located in holes in the sides of the beam in the shear force areas and measured the shear strains on the beam, which in turn were representative of the weight of the load.
U.S. Pat. No. 4,785,896, patented Nov. 22nd, 1985, by W. E. Jacobson, provided a load sensing structure for weighting which had a rectangular deck with four flexure members supporting the deck. Each flexure member had attachment portions secured to the deck and oppositely-facing attachment portions secured to a fixed platform. U-shaped flexure intermediate portions had parallel legs or beams that were connected to one another by a rigid base of the U, and these legs were also cantilever connected to the attachment portions. Two strain gauges on one such leg were so located that these gauges must sense tension and compression to indicate weight in the bridge circuit. If both detect tension or both compression no weight indication will occur.
U.S. Pat. No. 4,858,710, patented Aug. 22nd, 1989, by M. R. Krause, provided a load cell, in particular for weighing systems, comprising a deformable member, this deformable member has an aperture extending transversely of the direction of the force, which is closed by at least one disc-shaped wall which corresponds to the cross-section of the aperture and extends transversely of the axis of the aperture and serves to receive strain gauges which are deposited by means of a film technique. The deformable member is formed by two sub-members, which are welded together in a plane which extends transversely of the axis of the aperture. A wall is provided in the aperture of each sub-member, at least one wall being provided at its interior side with strain gauges.
While in the prior art, wherein the platform was connected to the base by flexural members, the number of transducers was reduced, the flexural members and the base must be carefully assembled and adjusted so that the single transducer will sense only the vertical load applied to the upper platform. Making such scales insensitive to off-centre loads had been found to be difficult, time consuming, and expensive. Substantially the same considerations applied to a prior art structure wherein the flexural members were an integral part of the single transducer. That type of structure was also sensitive to off-centre loads unless expensive mechanical or electronic adjustments are made to reduce the effects of off-centre loading.
One arrangement of the strain gauges in the prior art made it possible to measure at the output terminals of the Wheatstone bridge a signal which was theoretically proportional to the force applied on the weighing platform. In view of the fact that the resultant of the forces applied on the platform may be located at a point remote from the axis of flexure of the bar, the torsional stresses to which the bar may be subjected were liable to produce measurement errors which can attain 5 to 6%.
Another arrangement of the prior art made it possible to cancel the torsional stresses detected by the flexure-sensitive strain gauges. However, the arrangement of the four flexure-sensitive strain gauges on two opposite faces of the bar had the effect of introducing a considerable complication in the mass production of the weighing device and in the calibration of the device.
Yet another arrangement of the prior art was the so-called "floating" load cell.
A floating load cell is characterized by being freely supported off of its respective ends, typically by means of a pin which was positioned in a groove in the lower surface of the load cell. In such a load configuration, the ends of the load cell were not bolted or otherwise rigidly fixed in position, so that the load cell was free to deflect downwardly about the supporting pins. However, even in a floating load cell, it was still necessary to support the ends of the load cell and to prevent the load cell from lifting off the pin. Previous attempts to provide such support, however, had significant disadvantages. The support assemblies generally were bulky and relatively expensive. Further, and perhaps most importantly, the results of weigh platforms using such support assemblies for the load cells were characterized by severe inaccuracies, due to uneven surfaces on which the load cell was positioned and collection of debris in the support assemblies.
Weighing systems for on-board application have typically used a bending beam type load cell. However, such load cells in use experienced difficulties with breakage and cracking in particular areas of the load cell. Further, debris, including snow, ice, and mud, frequently accumulated in the area directly beneath the lower surface of the load cell beam, inhibiting the bending of the load cell, which in turn impaired the accuracy of the reading.
Still further, such load cells have proven to be vulnerable to moisture. The strain gauges which were used in the load cells were very sensitive to moisture, even to changes in humidity, and prior art on-board load cells have been difficult to protect against moisture, even with the application of state-of-the-art potting and/or sealing methods and materials. The above-described disadvantages result in a relatively high failure rate for conventional on-board load cells, which can impair the safe operation of the vehicle, and increase operational expense.
Inaccurate measurement of the applied load results unless the load is applied along a line which is virtually coaxial of the cell. That is, the known constructions are not immune from inaccuracies caused by laterally applied forces.
Others of the known load cells were subject to still other disadvantages, e.g., variations in reliability in response to changes in temperature, the inability to be adjusted for the accommodation of widely varying load factors, the likelihood of damage due to overloads, high manufacturing and maintenance costs, and complexity of assembly.
The prior art has already provided a wheel load indicator, which comprised a flat plate of elastic material which had measuring properties, the plate being adapted to be supported at two opposed lateral edges and including recesses disposed in pairs and formed webs between them to which strain gauges were attached for detecting shearing stresses.
In one known wheel load indicator of this kind the webs each were formed between circular enlargements of a pair of recesses, and strain gauges were adhered to the opposed walls of these enlargements, i.e., to the two opposed web walls. The recesses themselves were closed.
In another known wheel load indicator the recesses formed a web between them which were of slot-like design starting from the lateral edges of the plate. The strain gauges were adhered to the webs in the plane of the upper surface of the plate. With such an arrangement, nothing but bending stresses of a plate loaded by a wheel could be determined.
U.S. Pat. No. 4,616,723, patented Oct. 14, 1966, by L. Pietzasch et al, provided a wheel load indicator which provided that two closed cavities be arranged spaced from each other in each web and with walls extending in the cross-sectional direction of the plate, the strain gauges being attached to the walls to measure the sheer stresses within the webs. Upon introducing and adhering the strain gauges to the walls of the cavities, the latter could be closed hermetically in simple manner by using sealing plugs or the like. Thus, the measuring elements and their places of attachment as well as their electrical terminals were housed in surroundings protected from any harmful outside influences.
In this patent, the strain gauges were adhered to parallel, planar wall portions of the bore walls which were each located adjacent the neighbouring recesses, at uniform spacings from the lateral edge of the plate. Thus, each bore contained only one strain gauge. In a further modification, the plate was propped on the ground by having each web rest on a base member by way of a support, while the remaining plate areas were unsupported. Thus, the plate was supported along its two opposed lateral edges exclusively by the webs serving as "claws" and their supports.
In this patent, these supports preferably were arranged between the two bores of each web and were designed as point supports which supported the corresponding web on each of two base members extending below the lateral edges.
In this patent, the places of attachment of the strain gauges, as seen in the direction of the web width, were so selected that they coincided with the places of the least shearing deformation due to transverse bending of the webs.